1. Field of the Invention
This invention relates to the production of organic derivatives of cellulose, and particularly to a method for preparing organic derivatives of cellulose having a controlled degree of fibrillation or branching. More particularly, the invention relates to a method for producing cellulose acetate fibers having a degree of fibrillation greater than about 6. The fibrillated cellulose acetate may be used in the production of products typically constructed of cellulosic materials such as filter media, paper and paper products.
2. Background of the Invention
In the production of organic derivatives of cellulose, and especially organic esters of cellulose, such as cellulose acetate, cellulose formate, cellulose propionate and cellulose butyrate, the esterification of cellulose with an organic acid results in a solution of the derivative of cellulose in an acid solvent. For example, in making cellulose acetate, cellulose is acetylated by contacting a cellulosic material with acetic anhydride and a catalyst in the presence of a relatively large amount of acetic acid. The acetic acid dissolves the cellulose acetate that is formed, producing a very heavy and viscous solution, referred to herein as the "acid dope". Usually after hydrolysis, this solution of cellulose acetate is precipitated by adding water until the concentration of the acid reaches a point below which the acid will not hold the cellulose acetate in solution.
The isolation of cellulose acetate, or secondary cellulose materials, from organic solvent solutions, referred to herein as "solvent dope", has been extensively investigated. The known process for preparing cellulose acetate, i.e., a cellulose acetate with an approximate average degree of substitution of 2, with its acetylation and hydrolysis steps, results in a solution of the acetate in an acetic acid and water mixture. As noted above, the usual practice for precipitating cellulose acetate from the reaction mixtures obtained in the acetylation of cellulose, is to introduce a large volume of water into the reaction mixtures. A disadvantage of forming cellulose acetate in this way is that lumps of various sizes are precipitated. This adds to prolonged washing and subsequent handling procedures to purify the cellulose acetate.
Various techniques for isolating the cellulose acetate from that solution have resulted in cellulose acetate products in the form of powders, pellets or flakes. (See, for example, S. Gedon, R. Fengl, "Cellulose Ester, Organic". Kirk-Othmer Encyclopedia of Chemical Technology, 5.sup.th Edition, Volume 5, p. 510 (1993), John Wiley & Sons, Inc.) For various intermediate and end uses, the cellulose acetate products are generally dissolved in volatile organic solvents such as acetone and methyl ethyl ketone. The solutions can be placed on objects so that when the solvent evaporates, a thin film or coating of cellulose acetate remains on the object. Very concentrated solutions can be cast such that when the solvent evaporates, clear cellulose acetate sheets are the product. Forcing the concentrated cellulose acetate solutions through spinneret holes will result in a continuous fiber product. (See, for example, N. Eastman, et al., "Cellulose Acetate and Triacetate Fibers", Kirk-Othmer Encyclopedia of Chemical Technology, 3.sup.rd edition, Vol. 5, p. 105-108 (1979). John Wiley & Sons, Inc.) The fibers resulting from this spinning process are, in general, very long, regular, dense fibers. The fibers generally have a relatively constant diameter without kinks or curls in the fiber.
Attempts have been made to manufacture cellulose acetate fibers from the acetic acid dope with and without the use of such spinning technology. For example, U.S. Pat. No. 2,632,686 describes a process for wet spinning secondary cellulose acetate fibers from an acid dope and a process that will manufacture acetate films. The dope is extruded through a jet into an aqueous coagulating bath containing at least one metal salt as the coagulant medium. The examples in this patent show that the dope must be filtered and then deaerated for 6 hours before use. The fibers were made using 0.002 in. diameter spinnerets.
British Patent No. 790,039 describes a precipitation process in which the cellulose acetate-containing acid dope is extruded through apertures. The continuous filaments formed this way are allowed to fall freely through the air for a short distance before entering a hardening liquid. The filaments are allowed to fall through the liquid until they harden at least to the point where they will not coalesce or cohere on simple contact, then the fibers are deposited on a moving belt and carried to a cutter.
U.S. Pat. No. 1,456,781 describes a process for forcing an acetic acid solution of cellulose acetate through a filter screen and then through small orifices into a liquid capable of precipitating cellulose acetate in the form of an irregular mass of filaments.
U.S. Pat. No. 2,239,782 describes a similar process for the production of cellulose acetate fibers from acid dope. The equipment involves something similar to a horizontal continuous precipitator with the dope being moved from compartment to compartment while being diluted with precipitation liquids until the ester is precipitated as a fiber.
Although cellulose acetate fiber has been made for several decades, the conventional process requires that the cellulose acetate be first precipitated from the acetylation reaction solution, then re-dissolved in a volatile solvent, usually acetone or methylene chloride, to make a solvent dope from which the fiber is spun.
A number of methods have been described for preparing fibrets from cellulose acetate dopes. These usually involve precipitating the cellulose ester from a solvent ("dope") by the sudden addition of a "coagulant" which is miscible with the cellulose acetate solvent, but in which the cellulose acetate is insoluble. This is frequently accomplished by extruding the dope together with the coagulant into a region of high shear. While most of them mention that the fibers are fibrillated, there is no indication that the degree of fibrillation can be controlled or for that matter even measured.
In U.S. Pat. No. 3,842,007, cellulose acetate having a viscosity between 25 and 45 seconds, as measured according to ASTM D 1343-69, is dissolved in acetone to a solids concentration of 2-4% by weight. The polymer is subsequently precipitated by the addition of a large amount of water.
U.S. Pat. No. 3,961,007 discloses a method for making a fiber by feeding the cellulose acetate solution and the coagulating medium into a mixing T at velocities which create substantial turbulence and passing the resulting slurry immediately through a three blade impeller which shears the precipitated cellulose acetate to a surface area in the range of 35-55 m2/g. The fiber is claimed to resemble asbestos, and is useful as a filter medium.
U.S. Pat. No. 4,040,856 describes a process in which cellulose acetate, water, and a solvent are heated under pressure to form an emulsion which is "flashed" through a nozzle into a second vessel which is maintained at reduced pressure.
A number of references disclose making short, fine, cellulose acetate fibers by feeding a solution of the polymer dissolved in a solvent into a zone in which the solvent flashes. It is also known to make fibers by feeding a solution of dissolved polymer into a non-solvent under conditions of high shear. For example, in U.S. Pat. Nos. 3,342,921 and 3,441,473 small, opaque, cellulose ester fibers are produced by introducing a stream of a cellulose acetate-acetone-water solution through an atomizing spray nozzle into a stream of high pressure air. The resulting spray is allowed to fall into an agitated vessel which contains water, and the resulting fibers are recovered by filtration. The fibers are useful for increasing opacity of a paper.
U.S. Pat. No. 4,460,647 discloses a similar process, in which an inorganic opacifier such as titanium dioxide is added to the polymer solution prior to passing it through a spray nozzle.
U.S. Pat. Nos. 4,192,838; 5,175,276; 4,274,914; 4,283,186; and 5,071,599 all describe variations of the method where cellulose acetate dope is fed through a capillary into the throat of a venturi through which a coagulation liquid, such as water is passed.
EP 0 711 512 A2 discloses another approach to making small fibers where a cellulose acetate dope is fed through a nozzle, or series of nozzles, into a precipitating agent. The precipitated fiber is then subjected to a shearing force. The description of the apparatus shows the dope being fed through nozzles which are located concentrically in a pipe through which the coagulant is fed. A rotating cutting blade is located at or a short distance from the nozzle so that the dope stream is sheared into short lengths and fibrillated as it is extruded. The blade further introduces turbulence at the point at which the dope is coagulated.
U.S. Pat. No. 4,047,862 discloses a procedure in which the polymer is coagulated in a region of high shear by feeding a dope into a disk which contains nozzles on its periphery. The disk is rotated at a peripheral speed of at least 500 meters per second adjacent to a stationary annular wall located from 1/16 to 1/2 in. from the periphery of the disk. The disk and annular wall are immersed in the precipitating bath so that as the disk rotates it creates fibers via the shearing force created on the fiber by the precipitating bath.
Japanese SHO 52-96231 discloses a method for preparing acetylcellulose pulp. In the process a 5-20 weight percent acetylcellulose in a solvent of acetylcellulose is introduced into a stirred precipitating agent consisting of a 15-45 weight percent acetylcellulose solvent and water.
Japanese SHO 52-96208 describes a sheet consisting of 10-90 weight percent pulp produced by introducing, while stirring, a solution consisting of acetylcellulose with a concentration of 5-20 weight percent of acetylcellulose and a solvent of acetylcellulose into a precipitating solvent consisting of a solvent of acetylcellulose with 15-45 weight percent concentration of the solvent for acetylcellulose in water, and 90-100 weight percent short fibers, which are mixed, made into paper, pressed, and heated.
U.S. Pat. No. 5,705,631 describes the formation of fine cellulose acetate fibers by extruding the cellulose acetate-containing acid dope stream through an orifice or slit in a zone having a substantially laminar flow profile. This produces bands of fibers in which the individual fibers have a typical diameter of about 20 microns.
It is a characteristic of all of the foregoing processes, except that of U.S. Pat. No. 5,705,631, that the cellulose acetate dope is introduced directly into a coagulation zone under conditions of high shear. Control of the degree of fibrillation is apparently not possible, and the processes generally describe precipitating and re-dissolving polymer for the preparation of fibrets.
Despite such attempts to manufacture fibers of cellulose acetate from the acetic acid dope, a need remains for an economical and efficient process for making highly fibrillated cellulose acetate fibers. More importantly, a need exists for a process of manufacturing cellulose acetate fibers suitable for use in papermaking and other applications. The fibers should be prepared directly from the cellulose acetate-containing acid dope resulting from a typical cellulose acetylation and hydrolysis process.